(1) Field of the Invention
The present invention relates to an air-fuel ratio sensor for detecting an air-fuel ratio of an air-fuel mixture based on the oxygen concentration in the exhaust of an internal combustion engine, and more specifically, to the technique for controlling a heater for heating a sensor element of said air-fuel ratio sensor.
(2) Related Art of the Invention
In the prior art, there was known an electronically-controlled fuel injection apparatus of an internal combustion engine comprising an air-fuel ratio sensor for detecting an air-fuel ratio of an air-fuel mixture based on the oxygen concentration in the exhaust, wherein the quantity of fuel injection was feedback controlled so as to bring the air-fuel ratio of the air-fuel mixture close to the target air-fuel ratio (refer to Japanese Unexamined Patent Publication No. 60-240840).
The air-fuel ratio sensor mentioned above has a characteristic to vary the output thereof related to the oxygen concentration corresponding to the temperature of the sensor element. Therefore, in order to perform an accurate detection of the air-fuel ratio, it is requested that the temperature of the sensor element be maintained at the activation temperature or above.
Therefore, the apparatus disclosed for example in the Japanese Unexamined Patent Publication No. 60-235047 comprises an electric heater for heating the sensor element, and by supplying maximum power to the electric heater for a predetermined period after starting the engine, the sensor element is heated at a relatively early stage to the activation temperature or above, thereby enabling to perform an air-fuel ratio feedback control with high accuracy at an early stage. Moreover, in this the apparatus, the sensor element temperature is estimated from the temperature of the cooling water and the like of the engine, and the lower the temperature of the sensor element is, the more the period for supplying maximum power to the electric heater is increased.
In such apparatus, it was requested that the capacity of the electric heater be increased in order to activate the air-fuel ratio sensor at an early stage. However, if the capacity of the electric heater was increased and maximum power was supplied thereto from the start, there was a problem that the sensor element may experience a large temperature difference and that the sensor element may be damaged by the heat shock.